Translucent and radio-opaque glass ceramics

ABSTRACT

Radio-opaque glass ceramics are described which have high chemical durability and adjustable translucency, brightness and coefficient of thermal expansion.

The invention relates to translucent radio-opaque glass ceramics with high chemical durability and with adjustable translucency, brightness and coefficient of thermal expansion.

In the field of dentistry, glass ceramics are commonly used for restorative dental prosthesis, for instance for producing inlays, onlays, crowns, bridges and veneers. DE 44 23 793 discloses phosphosilicate glass ceramics with improved optical properties, adjustable translucency and optical brightness. These materials contain leucite as a main crystalline phase and have the advantage that their optical properties closely match, those of the natural teeth. While phosphate-free leucite glass ceramics usually contain pigments in order to adapt their appearance to that of the natural tooth, the phosphosilicate glass ceramics according to DE 44 23 793 do not require the addition of such components. Furthermore, the color brightness and color impression from the depth of the material surmounted that of other leucite materials by far.

The phosphosilicate glass ceramics of DE 44 23 793 contain calcium phosphate, with the CaO-content being 2.5 to 11.5 wt %. In combination with P₂O₅ and F the CaO-content causes the formation of needle-shaped calcium apatite, Ca₅(PO₄)₃F, alongside with leucite, KAlSi₂O₆. A disadvantage of these materials is that they show only little or no radio-opacity.

Radio-opacity of biomaterials is important in clinical application since it allows to detect the preparation border of a metal free veneer or crown versus the natural teeth by X-ray examination. The detection of the preparation border is important in clinical aftercare and in detecting undesired secondary caries and is therefore an integral part in the attempt to preserve the natural teeth substance of the patient.

From U.S. Pat. No. 5,952,235 it is known that apatite crystal phases of very small dimensions and amounts can be formed in glasses, so that the products look like glasses and are transparent. These glasses can be used in optical systems. However, these materials are not useful for dental applications since they lack opacity and translucency.

WO 91/12212 describes an apatite glass ceramic containing relatively high amounts of CaO and Al₂O₃ and a relatively small amount of SiO₂. These materials exhibit a high degree of white ness which renders them useful as glass ionomer cements. However, when using these materials as dental restorative materials they have the disadvantage that they are very opaque and that their translucency is not adjustable.

U.S. Pat. No. 5,236,495 discloses non-siliceous phosphate glass ceramics. These pure phosphate materials are unsatisfactory with regard to their chemical durability and are therefore not suitable as dental restorative materials.

The prior art materials show a couple of disadvantages and it is an object of the present invention to eliminate these disadvantages, i.e. to provide dental materials which are radio-opaque, have high chemical durability and adjustable translucency, brightness and coefficient of thermal expansion.

This object has surprisingly been achieved by a glass ceramic which comprise: Component Wt. % SiO₂ 46.0-58.0 P₂O₅ 2.0-6.0 Me₂O₃ 10.0-22.0 Me₂O 14.5-30.0 MeO  4.0-13.0 F 0.3-3.0

with Me₂O₃ being selected from: Component Wt. % Al₂O₃  8.0-16.0 Y₂O₃ 0.1-9.0 B₂O₃   0-9.0

with Me₂O being selected from: Component Wt. % Li₂O 0-3.0  Na₂O 0-9.0  K₂O 3.0-14.0   Rb₂O 0-12.5 Cs₂O 0-18.0

with MeO being selected from: Component Wt. % MgO 0-9.0 CaO 0-2.5 SrO  0-13.0 with the proviso that at least one of SrO or MgO ≧3.1 wt. % of CaO is 0.1 to 2.5 wt.-% and which has apatite as a main, crystalline phase.

It is preferred that the main crystalline phase is an apatite solid solution, in particular a Ca-apatite, wherein the Ca is totally or partially replaced by Sr and/or Mg. It is further, preferred that the main crystalline phase is Sr-apatite.

In the glass ceramics of the invention Me₂O₃, Me₂O and MeO are each selected from the compounds specified above. For instance, MeO may be a single compound, such as 13 wt. % SrO, or a mixture of compounds, such as 9 wt. % MgO and 4 wt. % SrO.

For the components exist the following preferred ranges which can be chosen independently of one another: Component Wt. % SiO₂ 46.5-58.0 P₂O₅ 2.2-6.0 Me₂O₃ 11.0-21.9 Me₂O 14.6-29.0 MeO  5.0-13.0 F 0.4-3.0

for Me₂O₃: Component Wt. % Al₂O₃  9.4-16.0 Y₂O₃ 0.2-9.0 B₂O₃ 0.1-8.8

for Me₂O: Component Wt. % Li₂O 0-1.0  Na₂O 0-8.7  K₂O 3.2-13.0   Rb₂O 0-12.4 Cs₂O 0-17.8

for MeO: Component Wt. % MgO 0-8.5 CaO 0-2.4 SrO 3.6-13.0 

For the components exist the following more preferred values which can also be chosen independently of one another: Component Wt. % SiO₂ 47.0-57.8 P₂O₅ 2.4-6.0 Me₂O₃ 11.5-21.8 Me₂O 14.7-28.5 MeO  6.0-13.0 F 0.5-3.0

for Me₂O₃: Component Wt. % Al₂O₃ 11.0-15.9 Y₂O₃ 0.3-9.0 B₂O₃ 0.3-8.6

for Me₂O: Component Wt. % Li₂O 0-0.5  Na₂O 0.1-8.4   K₂O 3.4-12.0   Rb₂O 0-12.0 Cs₂O 0-17.6

for MeO: Component Wt. % MgO 1.0-8.0 CaO 1.0-2.4 SrO  4.0-11.0

The radio-opaque glass ceramic as described above can additionally comprise ZrO₂, TiO₂, CeO₂. Preferred ranges for these compounds, which can be chosen independently of one another, are as follows: Component Wt. % ZrO₂ 0-1.0 TiO₂ 0-0.4 CeO₂ 0-1.0

The radio-opaque glass ceramic according to invention can further comprise one or more coloring or fluorescent metal oxides, selected from oxides of the group of metals consisting of Zr, Ta, Yb, Nb, Tb, La, Er, Pr, Ce, Ti, V, F, Mn and mixtures thereof. The total amount of these components is preferably less than 7% by weight, more preferably less than 5% by weight.

In the glass ceramic of the present invention apatite is the main crystalline phase. According to the present invention the term “apatite” is used for all crystal compositions which crystallize in the apatite crystalline structure. According to one embodiment the apatite phase may be a pure Ca-apatite phase of the formula Ca₅(PO₄)₃F. According to a preferred embodiment, of the invention the apatite phase is a phase of the formula Ca₅ (PO₄)₃F wherein the Ca is totally or partially replaced by Sr and/or Mg. Even more preferred are apatite phases wherein up to 50% of the calcium ions are replaced by Sr²⁺ and/or Mg²⁺. In the following, apatite phases wherein Ca ions are replaced by Sr and/or Mg ions will also be referred to as apatite solid solutions, apatite solid solution phases or simply solid solutions. Preferred apatite solid solution phases are Sr_(7.3)Ca_(2.7) (PO₄)₆F₂, or strontium fluorapatite, in particular Sr₅(PO₄)₃F.

Generally, the term solid solutions refers to mixed crystals, i.e. crystalline phases wherein ions of the crystal lattice have partially or totally been replaced by other ions.

It is further preferred that the crystals of the main crystal phase are needle shaped, crystal apatite phases having the form of needles with an average length of 0.1 to less than 10 μm are particularly preferred.

In addition to the main crystal phase the glass ceramics of the present invention preferably also contain one or more, in particular 1 to 3, further crystalline phases. In a particularly preferred embodiment the radio-opaque glass ceramic of the present invention has as a further crystalline phase KAlSi₂O₆, RbAlSi₂O₆ and/or CsAlSi₂O₆. A characterization/definition of KAlSi₂O₆ (leucite) can be found in JCPDS 38-1423, of RbAlSi₂O₆ (Rb-leucite) in JCPDS 85-1627 and of CsAlSi₂O₆ (pollucite) in JCPDS 88-0056. Such additional crystal phases are preferably of tetragonal or cubic modification. As pointed out above, Rb-leucite and pollucite can also be referred to as a solid solutions.

The glass ceramics according to the invention show an opacity which greatly resembles the opacity of the natural tooth. Preferably the ceramics have an opacity of about 0.3 to 0.7 (according to BS 512-1978) because it gives dental restorations a natural appearance and very good aesthetic properties.

In addition the glass ceramics of the invention have a high radio-opacity. The radio-opacity of dental materials is usually given as percentage of the radio-opacity of an aluminum plate having a thickness of 1 mm. The glass ceramics of the invention preferably have a radio-opacity of at least 100% Al, i.e. a glass ceramic plate with a thickness of 1 mm shows at least the same radio-opacity as a 1 mm aluminum plate. More preferably the radio-opaque glass ceramic of the present invention-have a radio-opacity of more than 200%, even more preferably of more than 250% and most preferably of more than 300% of the radio-opacity of a 1 mm Al plate (200% Al, 250% Al, 300% Al). By incorporation of cations having large ion radii, i.e. a covalent radius of about 1.6 Å, and relatively high, atom masses, i.e. atomic masses of at least 85, it is possible to increase the radio-opacity.

A radio-opaque glass ceramic as described above wherein at least one of the crystals of the main phase and at least one of the crystals of the secondary phase form a second phase is a preferred embodiment of the present invention.

FIG. 1 is a SEM image of a glass ceramic according to the invention comprising Sr-apatite as the main crystalline phase and leucite as a further crystalline phase (Example 1).

FIG. 2 shows the X-ray diffraction pattern of the glass ceramic of FIG. 1.

FIG. 3 is a SEM image of a glass ceramic according to the invention comprising Sr-apatite as the main crystalline phase and RbAlSi₂O₆ as a further crystalline phase (Example 8).

FIG. 4 shows the X-ray diffraction pattern of the glass ceramic of FIG. 3.

FIG. 5 is a SEM image of a glass ceramic according to invention comprising Sr-apatite as the main crystaline phase and pollucite as a further crystalline phase (Example 19).

FIG. 6 shows the X-ray diffraction pattern of the glass ceramic of FIG. 5.

FIG. 7 is a SEM image of a glass ceramic according to the invention comprising Sr—Ca-mixed apatite as the main crystalline phase and leucite as a further crystalline phase (Example 20).

FIG. 8 shows the X-ray diffraction pattern of the glass ceramic of FIG. 7.

In FIGS. 1, 3, 5 and 7 the main apatite phase forms needles. The secondary crystal phases are identifiable as etching patterns.

The leucite content of the glass ceramics according to the invention can be adjusted by varying the chemical composition of the starting glass, e.g. by varying the content of K₂O and/or Al₂O₃. By varying the leucite content it is possible to tailor the coefficient of thermal expansion to the desired application of the glass ceramic. The coefficient of thermal expansion of the radio-opaque glass ceramics can also be increased by the presence of RbAlSi₂O₆ and/or pollucite as a further crystalline phase.

The coefficient of thermal expansion of the radio-opaque glass ceramics according to the present invention can be adapted to a wide range of substrates. Therefore the glass ceramics are suitable as facing materials for many different substrates including other ceramics, such as ZrO₂ and its composites, Al₂O₃ and its composites, glass ceramic materials, such as lithium silicate, metal alloys based on Ag—Au, Au, Au—Pt, Ag—Pd, Pd, Co—Cr, as well as Ti and its alloys.

The glass ceramics of the invention are particularly suitable for coating or facing dental alloys with high contents of gold which have coefficients of thermal expansion within a range of 12 to 16×10⁻⁶ K⁻¹.

The glass ceramics of the invention are also particularly suitable for coating or facing materials having coefficients of thermal expansion in the range of about 6 to 10×10⁻⁶ K⁻¹ such as Al₂O₃ or ZrO₂ ceramics or titanium and its alloys.

In the oral cavity dental materials are permanently subjected e.g. to acidic liquids. The chemical durability of the materials is therefore another significant aspect. It is of utmost importance that ceramics used for dental purposes do not loose their brightness and roughness during use because otherwise they would invite plaque formation to occur. The radio-opaque glass ceramics of the present invention are characterized by high chemical stability. The ceramics preferably have a solubility value of less than 70.0 μg/cm², measured according to ISO 6872;1995, and more preferably of less than 50.0 μg/cm² It was surprisingly found that the chemical durability can be increased by the presence of Y₂O₃ and, therefore, Y₂O₃ containing ceramics are particularly preferred.

The radio-opaque glass ceramics according to the invention preferably have the form of a powder, a blank or a block. A particularly preferred embodiment of the present invention is a radio-opaque glass ceramic as defined above which has the shape of a dental restoration and it is most preferred that the dental restoration is an inlay, an onlay, a bridge, an abutment, a veneer, a facet, a crown, a partial crown, a framework or a coping.

The glass ceramics of the present invention are also suitable for the preparation of other glasses or glass ceramics. Glasses or glass ceramics which comprise a radio-opaque glass ceramic according to the invention form another aspect of the present invention. The radio-opaque glass ceramic according to the present invention can be combined with a wide variety of other glasses and/or glass ceramics. Such mixtures are also referred to as inorganic-inorganic composites.

Glasses and glass ceramics which can preferably be combined with the class ceramics of the present invention are disclosed in DE 43 14 817, DE 44 23 793, DE 44 23 794, DE 44 28 839, DE 196 47 739, DE 197 25 552 and DE 100 31 431. These glasses and glass ceramics are derived from silicate or borate or phosphate or alumina silicate systems. Preferred glasses and glass ceramics are derivable from the systems SiO₂—Al₂O₃—K₂O (with cubic or tetragonal leucite crystals), SiO₂—B₂O₃—Na₂O, alkali-silicate, alkali-zinc-silicate, silico-phosphate and/or SiO₂—ZrO₂.

Particularly preferred glasses and glass ceramics for combining with the glass ceramics of the present invention are defined as follows:

Low-temperature-sintering potassium-zinc-silicate glass (DE 100 31 431): Component Amount (in wt.-% SiO₂ 60.0-72.0 Li₂O 1.0-5.0 K₂O 10.0-23.0 ZnO  8.5-20.0

Low temperature-sintering apatite glass ceramic (DE 100 31 430): Component Amount (in wt.-% SiO₂ 56.0-65.0  Li₂O 1.8-5.3  K₂O 9.0-17.5 ZnO 9.0-16.0 CaO 3.5-10.5 P₂O₅ 2.0-6.0  F 0.5-1.0 

Translucent apatite glass ceramic (DE 197 25 555/DE 197 25 553): Component Amount (in wt.-% SiO₂ 45.0-70.0  Al₂O₃ 5.0-22.0 K₂O 3.0-8.5  Na₂O 4.0-13.0 CaO 1.5-11.0 P₂O₅ 0.5-6.5  F 0.1-2.5 

Alkali silicate glass (DE 197 25 552): Component Amount (in wt.-% SiO₂ 55.0-71.0  Al₂O₃ 5.0-16.0 B₂O₃ 0.2-10.0 K₂O 4.5-10.0 Na₂O 3.0-14.0

Sinterable lithium disilicate glass ceramic (DE 196 47 739): Component Amount (in wt.-% SiO₂ 57.0-80.0 Al₂O₃ 3.0-5.0 La₂O₃ 0.1-6.0 Li₂O 11.0-19.0

Alkali-zinc-silicate glass-ceramics and glasses (DE-44 28 839): Component Amount (in wt.-% SiO₂ 52.0-63.5  Me^(III) ₂O₃ 8.5-13.0 Na₂O 1.5-20.0 ZnO 2.0-8.0  Me^(II)O 2.5-6.5  TiO₂+ ZrO₂ 0.5-6.0 

ZrO₂—SiO₂— Glaskeramik (DE 44 23 794): Component Amount (in wt.-% SiO₂ 42.5-58.5 Li₂O  7.0-14.5 P₂O₅  4.0-13.5 ZrO₂ 15.0-28.0

Leucite-containing phosphosilicate glass ceramic (DE 44 23 793): Component Amount (in wt.-% SiO₂ 49.0-57.5  Al₂O₃ 11.4-21.0  P₂O₅ 0.5-5.5  CaO 2.5-11.5 K₂O 9.0-22.5 Na₂O 1.0-9.5  ZrO₂ 0.8-8.5  F 0.25-2.5 

Opalescent glass (DE 43 14 817): Component Amount (in wt.-% SiO₂ 48.0-66.0  Me^(III) ₂O₃ 5.0-20.0 Me^(I2)O 6.0-22.0 Me^(II)O 3.5-16.0 Me^(IV)O₂ 0.5-10.0 P₂O₅ 0.5-5.0 

Preferably 20 to 80 wt. % of one ore more radio-opaque glass ceramic according to the invention are-mixed with 80 to 20 wt. % of one or more of the above defined glasses or glass ceramics. By mixing such glasses with the radio-opaque glass ceramic according to the invention the coefficient of thermal expansion can be adjusted in a range of from 6 to 20×10⁻⁶ K⁻¹. Glasses and glass ceramics which are obtained by mixing one ore more radio-opaque glass ceramics according to the invention with one ore more other glasses or glass ceramics are also included in the present invention. The composition of these mixed glasses and glass ceramics can be calculated from the composition of the glasses and/or glass ceramics used for preparing the mixture and their percentages. These mixed glasses and glass ceramics are characterized by the presence of the apatite crystalline phase of the radio-opaque glass ceramics of the invention. According to the present invention glasses and glass ceramics which are free of BaO are preferred.

The development of the radio-opaque glass ceramic according to the present invention is based on the knowledge of the fundamental principles of controlled crystallization to give glass ceramics. The starting point is the controlled nucleation by phase separation which is based on a glass in glass phosphate phase separation.

It was surprisingly found that the process of the invention proceeds in such a way that the preferred ions, such as Sr²⁺-ions in CaO-free compositions or in compositions with small amounts of CaO, are enriched in the droplet phase. Until now the phenomenon of apatite formation in glass ceramics was only known for glasses having a content of CaO of at least 2.5 wt. %.

In addition to that it was surprisingly found that Sr²⁺-ions alone or in combination with small amounts of Ca²⁺ lead toe the formation of apatite nuclei in the droplet phase, the phase which is separated from the glass phase. This is often described as a phase separation phenomenon. The thermal treatment as described below results in a controlled crystallization of the phase-separated glasses, and apatite or apatite solid solution are obtained that grow in needle-shaped form. The formation of such apatites was verified by scanning electron microscopy (SEM). The verification of the exact crystallographic assignment was achieved by XRD. The micro structure is exemplarily shown in FIGS. 1, 3, 5 and 7, FIGS. 2, 4, 6 and 8 show XRD patterns of the glass ceramics shown in FIGS. 1, 3, 5 and 7.

Another aspect of the present invention is a process for the preparation of the radio-opaque glass ceramic as described above, which comprises

-   (a) producing a melt of a starting glass containing the components     of the glass ceramic, preferably at temperatures of 1200 to 1650°     C., more preferably between 1500 to 1550° C., -   (b) pouring the melt of the starting glass into water to give glass     granules, -   (c) optionally milling said glass granules to give a glass powder     having a mean particle size of 1 to 500 μm, preferably less than 150     μm, -   (d) subjecting said glass granules of step (b) or said glass powder     of step (c) to a heat treatment at a temperature of 700 to 1200° C.,     preferably at a temperature of 800 to 1100° C. for a period of 30     minutes to 6 hours, preferably 30 min. to 3 hours thus forming the     radio-opaque glass ceramic of the invention.

In step (a) the components needed to form the glass ceramic, such as e.g. carbonates, oxides, fluorides, and phosphates, are homogeneously mixed. Then the mixture is heated to a temperature within the range that is given above in order to form a melt of the starting glass.

Subsequently in step (b) the glass melt of step (a) is poured into water. By this so-called fritting step, glass granules are formed. By pouring the melted mixture into water small glass granules are obtained. Glass granules having a size within a range of 0.1 μm to <500 μm, in particular 0.1 μm to <150 μm are preferred.

In step (c) the glass granules are optionally milled. Usually they are grinded using standard mills to the desired particle size. The thus obtained glass powder preferably has a number average particle size of 1 to 500 μm, preferably less 200 μm and most preferably less than 100 μm.

In step (d) which follows step (b) or (c) the glass granules or glass powder is subjected to a thermal treatment in one or more steps at temperatures of 700 to 1200° C., preferably 800 to 1100° C., for a period of 30 minutes to 6 hours, preferably 30 minutes to 3 hours. The temperature should be higher than 800° C. in order to accelerate the formation of apatite crystals. The process of crystal growth takes place during step (d).

By SEM and XRD measurements it is possible to observe the apatite and apatite solid solutions as the main crystal phase. The size of the crystals can be adjusted by different heat treatments, i.e. one, two or more steps of heat treatment at different temperatures and different periods of time. In addition to the surprising finding of apatite and apatite solid solutions it was likewise found that by variation of the chemical composition within the above defined ranges and the use of different crystallization mechanisms a second crystalline phase, i.e. tetragonal leucite, Rb-leucite or Cs-leucite, can be precipitated.

By incorporation of small amounts of Ca²⁺ in combination with Sr²⁺ apatite solid solutions were formed. Also when using samples which contained magnesium, apatite needles were formed when small amounts of Ca²⁺ were present. By using the mechanism of controlled twofold crystallization (this means that both apatite and a second crystal phase like leucite crystallizes) a second crystalline phase was formed. This is due to a volume crystallization of the apatite in combination with a surface crystallization of the second phase. Thereby new combinations of properties of bio-materials are obtained, which are described in more detail in the experimental part.

The use of the radio-opaque glass ceramic as described above as a component of a glass or glass ceramic result in the formation of different micro structures. Such solid solution compositions are another aspect of the present invention. The main objective is to develop inorganic-inorganic composites with the radio-opaque glass ceramics according to the invention. Varying the part of the radio-opaque glass ceramic within that composite allows to adjust properties, such as the coefficient of thermal expansion and optical properties, e.g. the translucency.

The translucent and radio-opaque glass ceramic according to the invention is especially useful as a powder, resulting from step (d) of the above described manufacturing process. These powders can be used as such or in admixture with other glass or glass ceramic powders for coating different substrates as described before. This technique is characterized by applying a powder slurry or a wet powder material onto a substrate followed by a sintering step in order to fix the coating material to the substrate.

The radio-opaque glass ceramic powders according to the invention can also be used as a filler in organic-inorganic composite materials together with organic polymerizable monomers and an initiator system for the polymerization. The glass ceramic powder of the invention can be used as the sole filler or in combination with other filler components: When used as filler for organic-inorganic composites the glass ceramic powders of the invention preferably have a particle size of 0.1 μm to less than 250 μm, more preferably 0.1 μm to less than 10 μm. The glass ceramic powder is preferably used together with other filler types, such as rheological modifiers.

The glass ceramic powder of the invention is also useful, alone or in admixture with the above specified other glasses or glass ceramic powders, for pressing a blank which is sintered to form the crystalline structure. These blanks have typically a circular or a rectangular cross-section with different length or heights. These blanks can be further processed to shaped products. The overall dimensions of such a blank are preferably adapted to the manufacture of dental restorations.

The formation of crystalline structures is preferably achieved by a two-stage process. In the first step surface crystallization takes place which is followed by the volume crystallization. However, it is also possible to terminate this process after the first step such that only surface crystallization will take place. This stage of an incomplete crystallization leads to lower mechanical strength and thus blanks which are easier to further process in following manufacturing procedures are obtained.

Alternatively, blanks can be produced by pouring the glass melt of step (a) of the above process into a mould. The cast is then, maintained at temperature of about 800 to 1100° C. to achieve crystallization. By controlling time and temperature it is possible to cause the formation of different crystal phases. Blanks can also be manufactured by compacting the granules of step (b) or the powder of step (c) in combination with the subsequent heat treatment step (d).

The use of a blank produced as described above in pressing processes is described in EP 915 625 B1.

The use of a block as described above in CAD/CAM-processes is still another preferred embodiment of the present invention.

The present invention is also directed to radio-opaque glass ceramic products which are obtainable by the process as described above and in particular to radio-opaque glass ceramic products which are in the form of a dental restoration. Preferred dental restorations which can be produced from the radio-opaque glass ceramics according to the invention are inlays, onlays, crowns, bridges, partial crowns, a veneers, copings, abutments, facings, frameworks or facets.

The invention is explained in more detail on the basis of the following examples.

EXAMPLES Examples 1 to 30 Radio-Opaque Glass Ceramic

A total of 30 different radio-opaque glass ceramics according to the invention was produced as described above.

The composition of the 30 samples is given in Table I, together with the main crystal phases, which were analyzed via XRD (X-ray diffraction). Sr₅(PO₄)₃F is strontium apatite phase. (JCPDS 50-1744) and Sr_(7.3)Ca_(2.7)(PO₄)₆F₂ is Sr—Ca-mixed apatite phase (JCPDS 78-1715).

Samples of the different materials were examined after the heat treatment and cooling to room temperature with a Bruker-AXS diffractometer D 5005 operating with a Cu anode. The coefficient of thermal expansion was measured with the Bähr dilatometer. TABLE I Compositions and Main Crystalline Phases of Glass Ceramics 1 2 3 4 5 6 7 8 Component wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 52.3 51.9 51.4 50.8 49.2 49.1 47.7 48.6 P₂O₅ 3.8 3.8 3.8 3.7 3.6 3.6 3.5 3.6 B₂O₃ 0.3 0.3 0.3 0.3 0.3 3.0 0.3 0.3 Al₂O₃ 13.6 13.5 13.3 13.2 12.8 12.7 12.4 12.8 Y₂O₃ 0.1 1.0 1.9 3.1 6.1 6.1 9.0 0.5 ZrO₂ 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.8 TiO₂ 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 CeO₂ 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 Li₂O 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Na₂O 8.1 8.0 7.9 7.8 7.6 5.0 7.4 7.6 K₂O 10.3 10.2 10.1 10.0 9.7 9.6 9.4 3.4 Rb₂O — — — — — — — 12.4 Cs₂O — — — — — — — — MgO — — — — — — — — CaO — — — — — — — — SrO 8.8 8.7 8.6 8.5 8.3 8.3 8.0 8.3 F 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 main Sr-apatite Sr-apatite Sr-apatite Sr-apatite Sr-apatite Sr-apatite Sr-apatite Sr-apatite crystal leucite leucite leucite leucite leucite leucite leucite RbAlSi₂O₆ phases 9 10 11 12 13 14 15 16 Component wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 51.8 51.8 51.7 51.8 51.8 51.4 53.6 50.5 P₂O₅ 3.8 3.8 3.8 3.8 3.8 3.8 4.5 3.7 B₂O₃ 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Al₂O₃ 13.6 13.6 13.5 13.6 13.6 13.6 12.9 14.0 Y₂O₃ 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 ZrO₂ 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.8 TiO₂ 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 CeO₂ 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 Li₂O 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Na₂O 8.1 8.1 8.1 8.1 8.1 8.1 7.6 7.8 K₂O 10.2 10.1 10.0 10.2 10.1 10.0 9.7 12.0 Rb₂O — — — 0.2 0.4 0.6 — — Cs₂O 0.2 0.4 0.6 — — — — — MgO — — — — — — — — CaO — — — — — — — — SrO 8.8 8.8 8.8 8.8 8.8 8.8 8.3 8.6 F 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.7 main Sr-apatite Sr-apatite Sr-apatite Sr-apatite Sr-apatite Sr-apatite Sr-apatite Sr-apatite crystal leucite leucite leucite leucite leucite leucite leucite leucite phases 17 18 19 20 21 22 23 24 Component wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 52.7 49.5 45.7 53.0 49.5 48.3 53.2 57.3 P₂O₅ 2.5 3.7 3.4 3.9 3.9 3.8 3.9 3.4 B₂O₃ 0.3 0.3 0.3 0.3 8.4 8.2 — 0.3 Al₂O₃ 13.8 13.0 12.0 13.9 11.1 11.6 13.9 11.3 Y₂O₃ 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 ZrO₂ 0.9 0.8 0.8 0.9 — — — 0.8 TiO₂ 0.2 0.2 0.2 0.2 — — — 0.2 CeO₂ 0.7 0.7 0.7 0.8 — — — 0.7 Li₂O 0.2 0.2 0.2 0.2 — — — 0.2 Na₂O 8.2 7.7 7.1 8.3 7.5 6.9 8.3 7.8 K₂O 10.4 9.8 3.2 10.5 11.4 11.2 10.5 9.1 Rb₂O — — — — — — — — Cs₂O — — 17.6 — — — — — MgO — — — — — — — — CaO — — — 2.4 2.4 — — — SrO 8.9 13.0 7.8 4.5 4.6 8.8 9.0 7.8 F 0.7 0.6 0.6 0.7 0.7 0.7 0.7 0.6 main Sr-apatite Sr-apatite Sr-apatite SrCa- Sr7.3 Sr-apatite Sr-apatite Sr-apatite crystal leucite leucite pollucite apatite Ca2.7- leucite NaSrPO₄ phases leucite apatite 25 26 27 28 29 30 Component wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 49.6 50.2 49.0 52.0 50.7 48.8 P₂O₅ 6.0 3.7 3.6 3.8 3.9 4.0 B₂O₃ 0.3 0.3 7.9 0.3 0.3 0.3 Al₂O₃ 12.9 15.9 12.8 13.6 13.3 12.9 Y₂O₃ 0.5 0.5 0.5 4.0 4.0 4.1 ZrO₂ 0.8 0.9 — 0.9 0.9 0.9 TiO₂ 0.2 0.2 — 0.2 0.2 0.2 CeO₂ 0.7 0.7 — 0.8 0.8 0.8 Li₂O 0.2 0.2 — 0.2 0.2 0.2 Na₂O 7.7 3.4 7.6 8.0 7.9 7.5 K₂O 9.7 14.3 9.7 10.2 10.0 9.5 Rb₂O — — — — — — Cs₂O — — — — — — MgO — — — 3.1 5.0 8.0 CaO — — — 2.2 2.1 2.1 SrO 8.4 8.6 8.3 — — — F 3.0 0.7 0.6 0.7 0.7 0.7 main Sr-apatite Sr-apatite Sr-apatite apatite apatite apatite crystal leucite leucite leucite leucite phases

Example 31 Radioactivity of Radio-Opaque Glass Ceramics

The radioactivity of the composition of Example 1 and a powder of SrCO₃ as reference material were determined as follows:

30 to 40 g of powder of the samples were measured using a (gamma)spectrometer that was protected from background radiation. The detectors were calibrated by using a standard sand having the same geometry as the samples and being endowed with ¹⁵²Eu. By using such a standard sand the (gamma) self-absorption within the sample can be accounted for.

Some decay products of the ²³⁸U- and ²³²Th-decay rows can be of determined by (gamma)-spectroscopy. The activities of ²³⁸U and ²³²Th were calculated assuming radioactive equilibrium. The results are given in Table II. The calculation of the detection limit was performed according to DIN 25482.

As can be seen from the data of Table II, the use of SrCO₃ as raw material for the production of the glass ceramic is unobjectionable in regard of radioactivity. This is also true for the glass ceramic produced. The determination and calculation of the radioactivity of raw material and glass ceramic according to DIN 25482 showed that the values obtained were below the level of background radiation which is originating from the earth's crust and is about 0.03 Bq*g⁻¹ for ²³⁸U and ²³²Th respectively. According to ISO 6872 the limit for unsuitability for ²³⁸U-activity is 1.0 Bq*g⁻¹. TABLE II Radioactivity of the Glass Ceramic according to Example 1 Glass ceramic 1 Component wt.-% raw material SiO₂ 52.3 — P₂O₅ 3.8 — B₂O₃ 0.3 — Al₂O₃ 13.6 — Y₂O₃ 0.5 — ZrO₂ 0.9 — TiO₂ 0.2 — CeO₂ 0.7 — Li₂O 0.2 — Na₂O 8.1 — K₂O 10.3 — Rb₂O — — Cs₂O — — MgO — — CaO — — SrO 8.8 100% SrCO₃ F 0.7 — main crystal phases Sr-apatite leucite — 238U/Bq * g⁻¹ <0.03 0.036 ± 0.008 232Th/Bq * g⁻¹ <0.03 <0.03 Tg/° C. 518 561 main crystal phases Sr-apatite leucite Sr-apatite leucite

Examples 32 to 39 Thermal Expansion Coefficient of Radio-Opaque Glass Ceramics

To measure the coefficient of thermal expansion (alpha) a green body having the form of a bar was produced from a powder of the respective sample. Compositions according to examples 1 (32), 4 (33), 6 (34), 8 (35) 16 (36), 21 (37), 22 (38) and 26 (39) were used. The green bodies were sintered in a vacuum furnace, Furnace® P100 (Ivoclar Vivadent AG), using a heating rate of 60° C./min and a holding time of 1 min at the firing temperature as is given in Table III for the production of test pieces. Subsequently a glance firing was performed without vacuum, with a holding time of a 1 min at an end-temperature that was 20° C. above that of the preceding step. The coefficient of thermal expansion was that measured at the thus obtained test piece using a Bähr-dilatometer in the temperature range of 100 to 500° C. The coefficients of thermal expansion of eight samples are given in Table III.

Examples 40 to 47 Optical Properties of Radio-Opaque Glass Ceramics

Optical properties of glass ceramics were determined according to British Standard BS 5612;1978, sec. 8.11, by use of a Minolta-CR 300 apparatus. The determined values have contrast values between 0 and 1 in comparison to a black and a white reference sample. The value zero represents 100% transmission and 1 represents 100% absorption. The optical properties of eight samples are given in Table III. As can be seen from the data in Table III the glass ceramics according to the invention can be used as opacifying material in dental applications since they are opaque.

Examples 48 to 55 Glass Transition Temperature of Radio-Opaque Glass Ceramics

The glass transition temperature was measured using a Bähr dilatometer or a differential scanning calorimeter (DSC) by Netsch. The results for the eight samples are given in Table III.

Examples 56 to 59 Chemical Durability of Radio-Opaque Glass Ceramics

To determine the chemical durability—which in the field of dental products mainly is stability against acid—sample bodies of four different radio-opaque glass ceramics having compositions according to examples 1 (56), 2 (57), 3 (58) and 4 (59) and having a diameter of 12 mm and a thickness of 1 mm were prepared. This preparation was achieved by sintering a glass ceramic powder having a particle size less than of 90 μm in a Programat® P100. The powder was kept at the sinter temperature for 1 min. The test pieces were then kept for 1 min at the temperatures which are given in Table III as the firing/sinter temperature. After cooling down to room temperature the chemical durability of the thus obtained test samples was determined according to ISO 6872:1995., i.e. as loss of mass after 16 h in 4% acetic acid at 80° C., and the obtained data are presented in Table IV.

As can be seen from the data of Table IV the chemical durability of the radio-opaque glass ceramics of the invention is good, i.e. it is by far better than the limit value for dental materials which according to ISO 6872:1995 is 100 μg*cm⁻². The data also show that the chemical durability can be improved by incorporating Y₂O₃ (see table IV). A high amount of Y₂O₃ corresponds to a high chemical durability. TABLE III Physical Properties of Glass Ceramics 48 49 50 51 52 Component wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 51.9 50.8 49.1 48.6 50.5 P₂O₅ 3.8 3.7 3.6 3.6 3.7 B₂O₃ 0.3 0.3 3.0 0.3 0.3 Al₂O₃ 13.6 13.2 12.7 12.8 14.0 Y₂O₃ 0.5 3.1 6.1 0.5 0.5 ZrO₂ 0.9 0.8 0.8 0.8 0.8 TiO₂ 0.2 0.2 0.2 0.2 0.2 CeO₂ 0.7 0.7 0.7 0.7 0.7 Li₂O 0.2 0.2 0.2 0.2 0.2 Na₂O 8.1 7.8 5.0 7.8 7.8 K₂O 10.3 10.0 9.6 8.4 12.0 Rb₂O — — — — — Cs₂O — — — — — MgO — — — — — CaO — — — — — SrO 8.8 8.5 8.3 8.3 8.6 F 0.7 0.7 0.6 0.6 0.7 melting 1550° C./2 h 1550° C./2 h 1550° C./2 h 1550° C./2 h 1550° C./2 h conditions optical Opalescent Opalescent Opalescent Opalescent Opalescent property after quenching Heat treatment 800° C./1 h 800° C./1 h 800° C./1 h 800° C./1 h 800° C./1 h and 1050° C.-/ and 1050° C./ and 1050° C./ and and 1050° C.-/ 0.5 h 0.5 h 0.5 h 1050° C.-/ 0.5 h 0.5 h optical property translucent translucent Translucent Translucent Translucent after tempering light light yellowish yellowish firing 920 980 990 1040 1080 temp./° C. C.T.E.* 13.5 14.2 12.6 13.5 15.9 100-500/ 10⁻⁶K⁻¹ Tg/° C. 518 561 601 512 507 main crystal Sr-apatite Sr-apatite Sr-apatite Sr-apatite Sr-apatite phases leucite leucite leucite Rb-leucite leucite 53 54 55 Component wt.-% wt.-% wt.-% SiO₂ 49.5 48.3 50.2 P₂O₅ 3.9 3.8 3.7 B₂O₃ 8.4 8.2 0.3 Al₂O₃ 11.1 11.6 15.9 Y₂O₃ 0.5 0.5 0.5 ZrO₂ — — 0.9 TiO₂ — — 0.2 CeO₂ — — 0.7 Li₂O — — 0.2 Na₂O 7.5 6.9 3.4 K₂O 11.4 11.2 14.3 Rb₂O — — — Cs₂O — — — MgO — — — CaO 2.5 — — SrO 4.8 8.8 8.6 F 0.7 0.7 0.7 melting 1550° C./2 h 1550° C./2 h 1550° C./2 h conditions optical Opalescent Opalescent Opalescent property after quenching Heat treatment 800° C./1 h 800° C./1 h 800° C./1 h and 1050° C.-/ and 1050° C.-/ and 1050° C.-/ 0.5 h 0.5 h 0.5 h optical property Translucent Translucent translucent after tempering firing 860 860 >1200 temp./° C. C.T.E.* 10.1 9.8 19.7 100-500/ 10⁻⁶K⁻¹ Tg/° C. 568 575 594 main crystal Sr—Ca- Sr-apatite Sr-apatite phases apatite leucite *Coefficient of thermal expansion

TABLE IV Chemical Durability of Glass Ceramics 56 57 58 59 Component wt.-% wt.-% wt.-% wt.-% SiO₂ 52.3 51.9 51.4 50.8 P₂O₅ 3.8 3.8 3.8 3.7 B₂O₃ 0.3 0.3 0.3 0.3 Al₂O₃ 13.6 13.5 13.3 13.2 Y₂O₃ 0.1 1.0 1.9 3.1 ZrO₂ 0.9 0.9 0.9 0.8 TiO₂ 0.2 0.2 0.2 0.2 CeO₂ 0.7 0.7 0.7 0.7 Li₂O 0.2 0.2 0.2 0.2 Na₂O 8.1 8.0 7.9 7.8 K₂O 10.3 10.2 10.1 10.0 Rb₂O — — — — Cs₂O — — — — MgO — — — — CaO — — — — SrO 8.8 8.7 8.6 8.5 F 0.7 0.7 0.7 0.7 chemical durability/^(μ)g * cm−² 67.0 57.8 37.1 24.1

Examples 60 to 64 Radio-Opacity of Radio-Opaque Glass Ceramics

The radio-opacity of the radio-opaque glass ceramics of the present invention was determined for five samples having compositions according to those of examples 5 (60), 7 (61), 8 (62), 16 (63) and 17 (64). The measurement of the radio-opacity was performed as follows:

According to the method of determining the radio-opacity of dental composite materials (ISO 4049) glass ceramic probes of 1 mm thickness were measured (device: Oralix DC by Gentix) and evaluated with Prepress RP 115 by Shamrock.

The reference for the value of radio-opacity is an aluminum-standard stair (Al 99.5%) of 1-5 mm thickness.

The results are summarized in Table V. TABLE V Radio-opacity of Glass Ceramics 60 61 62 63 64 Component wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 49.2 47.7 48.6 50.5 52.7 P₂O₅ 3.6 3.5 3.6 3.7 2.5 B₂O₃ 0.3 0.3 0.3 0.3 0.3 Al₂O₃ 12.8 12.4 12.8 14.0 13.8 Y₂O₃ 6.1 9.0 0.5 0.5 0.5 ZrO₂ 0.8 0.8 0.8 0.8 0.8 TiO₂ 0.2 0.2 0.2 0.2 0.2 CeO₂ 0.7 0.7 0.7 0.7 0.7 Li₂O 0.2 0.2 0.2 0.2 0.2 Na₂O 7.6 7.4 7.6 7.8 8.2 K₂O 9.7 9.4 3.4 12.0 10.4 Rb₂O — — 12.4 — — Cs₂O — — — — — MgO — — — — — CaO — — — — — SrO 8.3 8.0 8.3 8.6 8.9 F 0.6 0.6 0.6 0.7 0.7 radio-opacity 250 394 377 255 252 (% Al)

It can be seen from the data in Table V that the radio-opaque glass ceramics according to the invention exhibit values for the radio-opacity that are well above 100% Al.

Examples 65 to 74 Inorganic-Inorganic Composites Containing Radio-Opaque Glass Ceramics

Examples 65 to 74 demonstrate inorganic-inorganic composites with a radio-opaque glass ceramic according to Example 4 and with different amounts of an alkali-silicate glass, the composition of which is given in Table VI. Example 4 was mixed with different amounts of an alkali-silicate-glass (examples 65-69) and example 22 with different amounts of a boro-silicate-glass (examples 70-74). They were mixed in a so-called Turbula-mixer for about 10 to 60 minutes, preferred 30 minutes and sintered at temperatures, given in table VI to form an inorganic-inorganic composite glass ceramic.

The glass transition temperature was measured according to the procedure that was described for examples 48 to 55, the chemical durability according to the method described for examples 56 to 59 and optical properties. The optical properties of translucency (values between 0 and 1) were determined according to BS 512;1978.

The definitions of the characteristic color values (L, a, b) are given in BS 5612;1978.

By preparing the inorganic-inorganic composites containing the radio-opaque glass ceramic another possibility for adjusting the translucence as well as the coefficient of thermal expansion of the resulting products is established as can be seen from the data shown in Table VI. TABLE VI Inorganic-inorganic composite materials 66 (75 67 (50% 68 (25% Expl. 4: Expl. 4: Expl. 4: 69 100% 25% alkali- 50% alkali- 75% alkali- alkali- 65 (100% silicate- silicate- silicate- silicate- Expl. 4) glass) glass) glass) glass Component wt.-% wt.-% wt.-% wt.-% Wt.-% SiO₂ 50.8 52.2 53.6 55.0 56.4 P₂O₅ 3.7 2.8 1.9 0.9 — B₂O₃ 0.3 2.2 4.1 6.0 7.83 Al₂O₃ 13.2 13.1 12.9 12.8 12.7 Y₂O₃ 3.1 2.4 1.6 0.8 — ZrO₂ 0.8 0.9 1.0 1.0 1.1 TiO₂ 0.2 0.2 0.1 0.1 — CeO₂ 0.7 0.7 0.6 0.6 0.5 Li₂O 0.2 0.2 0.2 0.2 0.2 Na₂O 7.8 7.6 7.4 7.2 7.0 K₂O 10.0 10.1 10.3 10.4 10.6 Rb₂O — — — — — Cs₂O — — — — — CaO — 0.3 0.5 0.8 1.1 SrO 8.5 6.4 4.3 2.1 — ZnO — 0.5 1.0 1.5 2.0 F 0.7 0.7 0.7 0.7 0.7 TEC_(100-500° C.)/ 14.2 12.7 11.6 10.3 8.6 10⁻⁵ · K⁻¹ Tg/° C. 561 550 535 534 542 firing temp./ 980 980 940 870 850 ° C. chem. durability./ 24.1 29.4 32.1 37.9 33.1 ^(μ)g/cm² optical properties L 90.75 90.51 89.01 85.25 75.12 a −1.47 −1.57 −1.77 −2.07 −0.33 b 3.90 2.34 1.23 −0.18 3.62 CR/% 88.51 88.14 74.31 57.32 8.52 71 (60% 72 (40% 73 (20% Expl. 22: Expl. 22: Expl. 22: 74 (100% 40% Boro- 60% Boro- 80% Boro- Boro- 70 (100% Silicate- Silicate- Silicate- Silicate- Component Expl. 22) Glass) Glass) Glass) Glass) Oxide wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 48.3 59.5 65.1 70.7 76.3 P₂O₅ 3.8 2.3 1.5 0.8 B₂O₃ 8.2 8.9 9.2 9.6 9.9 Al₂O₃ 11.6 9.1 7.9 6.6 5.4 Y₂O₃ 0.5 0.3 0.2 0.1 — ZrO₂ — — — — — TiO₂ — — — — — CeO₂ — — — — — Li₂O — — — — — Na₂O 6.9 6.8 6.8 6.7 6.7 K₂O 11.2 6.8 4.6 2.4 0.2 Rb₂O — — — — — Cs₂O — — — — — CaO — 0.6 0.9 1.2 1.5 SrO 8.8 5.3 3.5 1.8 — ZnO — — — — — F 0.7 0.4 0.3 0.1 — C.T.E _(100-500° C.)/ 9.81 7.66 6.76 6.09 not 10⁻⁶ K⁻¹ measured Tg/° C. 575 577 582 580 not measured Firing temperature/ 860 880 870 890 not ° C. measured Chem. durability/ 132.4 79.2 56.4 37.8 not ^(μ)g * cm⁻² measured Optical properties L not 89.49 87.56 85.63 not measured measured a not −0.28 −0.51 −1.16 not measured measured b not 1.73 1.73 0.94 not measured measured CR/% not 87.07 80.93 60.11 not measured measured 

1. Translucent and radio-opaque glass ceramic which comprises Component Wt. % SiO₂ 46.0-58.0 P₂O₅ 2.0-6.0 Me₂O₃ 10.0-22.0 Me₂O 14.5-30.0 MeO  4.0-13.0 F  0.3-3.0,

with Me₂O₃ being selected from Component Wt. % Al₂O₃ 8.0-16.0 Y₂O₃ 0.1-9.0  B₂O₃   0-9.0,

Me₂O being selected from Component Wt. % Li₂O 0-3.0 Na₂O 0-9.0 K₂O 3.0-14.0  Rb₂O  0-12.5 Cs₂O   0-18.0,

MeO being selected from Component Wt. % MgO 0-9.0 CaO 0-2.5 SrO  0-13.0

with the proviso that at least one of SrO or MgO ≧3.1 wt. % if CaO is 0.1 to 2.5 wt.-%, and which has apatite as a main crystalline phase.
 2. Glass ceramic according to claim 1, wherein the main crystalline phase is an apatite solid solution.
 3. Glass ceramic according to claim 2, wherein the apatite solid solution is Ca-apatite wherein the Ca is totally or partially replaced by Sr and/or Mg.
 4. Glass ceramic according to claim 1, wherein the main crystalline phase is Sr-apatite.
 5. Glass ceramic according to claim 1, which comprises the components, independently of one another, in the following amounts Component Wt. % SiO₂ 46.5-58.0  P₂O₅ 2.2-6.0  Me₂O₃ 11.0-21.9  Me₂O 14.6-29.0  MeO 5.0-13.0 F 0.4-3.0  Al₂O₃ 9.4-16.0 Y₂O₃ 0.2-9.0  B₂O₃ 0.1-8.8  Li₂O  0-1.0 Na₂O  0-8.7 K₂O 3.2-13.0 Rb₂O   0-12.4 Cs₂O   0-17.8 MgO  0-8.5 CaO  0-2.4 SrO  3.6-13.0.


6. Glass ceramic according to claim 5 which comprises the components, independently of one another, in the following amounts Component Wt. % SiO₂ 47.0-57.8  P₂O₅ 2.4-6.0  Me₂O₃ 11.5-21.8  Me₂O 14.7-28.5  MeO 6.0-13.0 F 0.5-3.0  Al₂O₃ 11.0-15.9  Y₂O₃ 0.3-9.0  B₂O₃ 0.3-8.6  Li₂O  0-0.5 Na₂O 0.1-8.4  K₂O 3.4-12.0 Rb₂O   0-12.0 Cs₂O   0-17.6 MgO 1.0-8.0  CaO 1.0-2.4  SrO  4.0-11.0.


7. Glass ceramic according to claim 1, which further comprises one or more coloring or fluorescent metal oxides, selected from oxides of the group of metals consisting of Zr, Ta, Yb, Nb, Th, La, Er, Pr, Ce, Ti, V, Fe and Mn.
 8. Glass ceramic according to claim 1, which further comprises one or more of the following components: Component Wt. % ZrO₂ 0-1.0 TiO₂ 0-0.4 CeO₂ 0-1.0.


9. Glass ceramic according to claim 1, wherein the main crystalline phase is Sr_(7.3)Ca_(2.7)(PO₄)₆F₂ or Sr₅(PO₄)₃F.
 10. Glass ceramic according to claim 1, which contains one or more additional crystal phases.
 11. Glass ceramic according to claim 10, which has as a further crystalline phase KAlSi₂O₆, RbAlSi₂O₆ or CsAlSi₂O₆.
 12. Glass ceramic according to claim 1, wherein the crystals of the main crystal phase are needle-like shaped.
 13. Glass ceramic according to claim 12, wherein the needles have a length of less than 10 μm.
 14. Glass ceramic according to claim 1, having a radio-opacity of at least 100% Al.
 15. Glass ceramic according to claim 14 having a radio-opacity of more than 200% Al.
 16. Glass ceramic according to claim 15 having a radio-opacity of more than 250% Al.
 17. Glass ceramic according to claim 10, wherein the additional crystals are of needle-like or plate-like shape.
 18. Glass ceramic according to claim 1, wherein at least one of the one or more crystal phases or at least one of the one or more additional crystal phases are a solid solution.
 19. Glass ceramic according to claim 1, which has a chemical durability value of less than 100.0 μg/cm².
 20. Glass ceramic according to claim 19, which has a chemical durability value of less than 70.0 μg/cm².
 21. Glass ceramic according to claim 1, which is a powder or has the form of a blank or a block.
 22. A process for the preparation of a translucent and radio-opaque glass ceramic, which comprises (a) producing a melt of a starting glass containing the components of the glass ceramic according to claim 1 at temperatures of 1200 to 1650° C., (b) pouring the melt of the starting glass into water to obtain glass granules, (c) optionally milling said glass granules to obtain a glass powder preferably having a mean particle size of 1 to 500 μm and (d) subjecting said glass granules of step (b) or said glass powder of step (c) to a heat treatment at a temperature of 800 to 1100° C. for a period of 30 minutes to 6 hours, to form the translucent and radio-opaque glass ceramic.
 23. (canceled)
 24. Process according to claim 22, wherein the starting glass further comprises coloring or fluorescent components.
 25. Process for the preparation of a translucent and radio-opaque glass ceramic in form of a blank, which comprises (a′) producing a melt of a starting glass containing the components of the glass ceramic according to claim 1 at temperatures of 1200 to 1650° C., (b′) pouring the melt of the starting glass into a mould to form a starting glass blank, (c′) optionally cooling the glass blank to room temperature, (d′) subjecting said blank of step (b′) or step (c′) to a heat treatment at a temperature of 800 to 1100° C. for a period of 30 minutes to 6 hours, to form the translucent and radio-opaque glass ceramic.
 26. Process according to claim 25, wherein step (b′) is directly followed by the heat treatment according to step (d′).
 27. A method to for preparing a dental layering material comprising mixing of a translucent and radio-opaque glass ceramic according to claim 1 with one or more glasses or glass ceramics selected from the group consisting of SiO₂—Al₂O₃—K₂O based glasses and glass ceramics (with cubic or tetragonal leucite crystals), SiO₂—B₂O₃—Na₂O based glasses and glass ceramics, alkali-silicate based glasses and glass ceramics, alkali-zinc-silicate silico-phosphate and/or SiO₂—ZrO₂ based glasses and glass ceramics.
 28. Method according to claim 27, wherein the optical properties and the coefficient thermal expansion are adjustable.
 29. Method according to claim 27, wherein the layering material has a coefficient of thermal expansion in the range between 6 to 20×10⁻⁶ K⁻¹, measured in the temperature range of 100 to 500° C.
 30. A method of producing a dental restoration comprising the steps of (a) mixing a glass ceramic according to claim 1 (b) applying the mixture of step (a) to a substrate and (c) firing the coated structure of step (b) to fix the layering material to the substrate.
 31. Method according to claim 30, wherein the substrate is a substructure made of metal, metal alloy, glass, glass ceramic or ceramic.
 32. Method according to claim 31, wherein the dental restoration is an inlay, an onlay, a bridge, an abutment, a facing, a veneer, a facet, a crown, a partial crown, a framework or a coping.
 33. (canceled)
 34. A process for manufacturing a dental restoration comprising (a) providing the translucent and radio-opaque glass ceramic according to claim 21, (b) forming the glass-ceramic into a block or blank, (c) shaping said block or blank into a dental restoration.
 35. Process according to claim 34, wherein the shaping is achieved by a machining process.
 36. Process according to claim 35, wherein the machining process is a computer controlled process.
 37. Process according to claim 36, wherein this process is a CAD/CAM process.
 38. Process according to claim 34, wherein the shaping is achieved by a pressing process.
 39. Process according to claim 38, which further comprises (d) preparing a mould for a dental framework based on a model of the restoration by the wax-up technology, (e) embedding the waxed-up framework in an embedding material, (f) burning out the wax of the embedding material, (c′) heating up the blank and transforming it into a viscous state and (c″) pressing the viscous glass ceramic into the mould to form the dental framework.
 40. A method of preparing a coated dental framework which comprises (a) preparing a model of a dental restoration from a framework with a waxed-up coating, (b) embedding the model in an embedding material, (c) burning out the wax from the model, (d) providing the translucent and radio-opaque glass ceramic according to claim 21, (e) forming the glass-ceramic into a block or a blank, (f) heating up the blank and transforming it into a viscous state and (g) pressing the viscous glass ceramic into the mould to coat the framework.
 41. Process according to claim 22, wherein the melt is produced at temperatures between 1500 and 1500° C.
 42. Process according to claim 22, wherein the glass powder mean particle size is less than 150 μm.
 43. Process according to claim 22, wherein said heat treatment of step (d) is for a period of 30 minutes to 3 hours.
 44. Process according to claim 25, wherein the melt is produced at temperatures between 1500 and 1550° C.
 45. Process according to claim 25, wherein said heat treatment of step (d) is for a period of 30 minutes to 3 hours.
 46. A method of producing a dental restoration comprising the steps of (a) mixing a layering material obtainable by the method according claim 27 with a liquid, (b) applying the mixture of step (a) to a substrate and (c) firing the coated structure of step (b) to fix the layering material to the substrate. 